Ion mobility spectrometry (IMS) is a fast, high-resolution gas-phase ion separation technique in which ions travel a known distance through a drift cell in an environment of a known gas pressure and composition. The ions are produced from a sample in an ion source and travel through the drift cell under the influence of a DC voltage gradient. During this travel, the ions become separated based on their different collision cross-sections, which can be correlated to their differing mobilities through the drift gas. From the drift cell the ions arrive at an ion detector that counts the separated ions, enabling the production of peak information useful for distinguishing among the different analyte ion species detected. An IMS system may be coupled online with a mass spectrometry (MS) system, which utilizes a mass analyzer to separate ions based on their differing mass-to-charge ratios (or m/z ratios, or more simply “masses”). In particular, an IMS system may be coupled with a time-of-flight (TOF) MS system that provides fast, high-resolution mass analysis. In the combined IMS-TOF MS system, ions are separated by mobility prior to being transmitted into the TOF MS where they are mass-resolved based on their flight times to the detector. Performing the two separation techniques in tandem is particularly useful in the analysis of complex chemical mixtures, including biopolymers such as polynucleotides, proteins, carbohydrates and the like, as the added dimension provided by the IM separation may help to mass-resolve large ions that are different from each other but present overlapping mass peaks. This two-dimensional separation technique may be further enhanced by coupling it with liquid chromatography (LC) techniques.
In known systems, IMS instrumentation has been coupled to TOF MS instrumentation by means of segmented quadrupole ion guides with multiple differentially pumped pressure regions, skimmer interfaces, and RF-only hexapole ion guides. These interface components can limit the IMS resolution and ion transmission across the mass range, and can also complicate the instrument design and manufacturing.
Therefore, there is a need for interfacing IMS instrumentation with TOF MS instrumentation in a way that optimizes IMS resolution and ion transmission.